Improved joint subcarrier and power allocation to enhance the throughputs and user fairness in indoor OFDM-NOMA VLC systems.

Orthogonal frequency division multiplexing non-orthogonal multiple access (OFDM-NOMA) is a promising multi-user access scheme in indoor visible light communication (VLC) systems. In this paper, we propose three novel joint subcarrier and power allocation algorithms in OFDM-NOMA-VLC to improve the throughputs and/or user fairness. These three proposed algorithms address the requirements in fairness-throughput-balanced (FTB), fairness-first (FF), and throughput-first (TF) scenarios, respectively.

Diversity-reception UWOC system using solar panel array and maximum ratio combining.

We demonstrated a diversity-reception lens-free underwater wireless optical communication system employing a 2×2 solar panel array as detectors. The respective relationships between solar panel sizes and photocurrents, output voltages, system bandwidths were studied theoretically and experimentally. The signals output from the array were combined via maximum ratio combining in order to improve the signal quality.

47-kbit/s RGB-LED-based optical camera communication based on 2D-CNN and XOR-based data loss compensation.

In an RGB-LED-based optical camera communication (OCC) system, the inter-symbol interference and inter-channel interference deteriorate the transmission performance considerably. In this paper, a two-dimensional CNN structure is proposed for data recovery by learning features between color channels and neighboring symbols in the rolling shutter based OCC system under random data transmission. Moreover, we further propose an XOR-based data loss compensation method to realize 21% data rate improvement by restoring the lost data during the transmission.

Performance-enhanced gigabit/s MIMO-OFDM visible light communications using CSI-free/dependent precoding techniques.

In this paper, we propose two digital signal processing (DSP) techniques, the orthogonal circulant matrix transform (OCT) technique and the singular value decomposition (SVD)-based adaptive loading, to reduce the bit error rate (BER) of multiple-input-multiple-output orthogonal frequency division multiplexing (MIMO-OFDM)-based visible light communication (VLC) systems, without and with using the channel state information (CSI), respectively. A gigabit/s 2 × 2 MIMO-OFDM VLC system under ~100-MHz system bandwidth, with both symmetrical and asymmetrical MIMO setups, is demonstrated. It is shown that both techniques can attain outstanding BER reduction regardless of the transceivers' geometrical distributions.

On the study of the relation between linear/nonlinear PAPR reduction and transmission performance for OFDM-based VLC systems.

The relation between the peak-to-average-power-ratio (PAPR) reduction schemes and the transmission performance of the orthogonal frequency division multiplexing (OFDM) based visible light communication (VLC) system is experimentally investigated. The linear selective mapping (SLM) scheme and the nonlinear logarithmic companding scheme are optimized by considering both PAPR reduction and bit error rate (BER) performance. It is demonstrated that the logarithmic companding scheme, albeit providing larger PAPR reduction when compared to the SLM scheme, may result in worse BER performance due to the additional noise induced in its expanding process.

Experimental investigation of multi-band OCT precoding for OFDM-based visible light communications.

In this paper, we propose and experimentally demonstrate a channel-independent multi-band orthogonal circulant matrix transform (MB-OCT) precoding, to efficiently combat the severe frequency-selective fading of visible light communications (VLC). The proposed MB-OCT precoding exhibits an attractive ladder-like signal-to-noise-ratio (SNR) profile, thus can significantly reduce system BER by applying different quadrature amplitude modulation (QAM) level to different sub-bands. The impacts of sub-band number, signal bandwidth, and length of cyclic prefix (CP) on bit error rate (BER) of the VLC system are investigated.

Adaptively loaded IM/DD optical OFDM based on set-partitioned QAM formats.

We investigate the constellation design and symbol error rate (SER) of set-partitioned (SP) quadrature amplitude modulation (QAM) formats. Based on the SER analysis, we derive the adaptive bit and power loading algorithm for SP QAM based intensity-modulation direct-detection (IM/DD) orthogonal frequency division multiplexing (OFDM). We experimentally show that the proposed system significantly outperforms the conventional adaptively-loaded IM/DD OFDM and can increase the data rate from 36 Gbit/s to 42 Gbit/s in the presence of severe dispersion-induced spectral nulls after 40-km single-mode fiber.

Toward user mobility for OFDM-based visible light communications.

We propose and experimentally demonstrate a mobile visible light communications (mobi-VLC) transmission system. The impact of user mobility on the performance of the mobi-VLC system is characterized, and we propose the use of the channel-independent orthogonal circulant matrix transform (OCT) precoding to combat the packet loss performance degradation induced by mobility. A mobile user terminal is used to detect the signal from a blue laser placed at 1 m away from the moving track.

Measurement method of the relative propagation delay of two signals based on modified caliper ruler.

A novel measurement technique for the relative propagation delay of two signals based on a caliper ruler is proposed. The technique is applied to the chromatic dispersion measurements of single-mode fiber (SMF), dispersion-compensation fiber, and dispersion-shifted fiber. Compared to the conventional time-of-flight method, dispersion can be measured even when the delay difference is smaller than the measurement pulse's width.

Fast imaging of high-resolution two-dimensional effective attenuation profile from diffuse reflectance.

Biological tissue is non-homogeneous in nature. It is difficult to measure its optical properties due to non-uniformity throughout the tissue being tested. To obtain the spatial distribution of optical parameters, conventional approaches use an array of light sources and detectors to reconstruct the image, thus, there is very limited spatial resolution.

Robust optical signal-to-noise ratio monitoring scheme using a phase-modulator-embedded fiber loop mirror.

We propose and experimentally demonstrate a novel in-band optical signal-to-noise ratio (OSNR) monitoring technique using a phase-modulator-embedded fiber loop mirror. This technique measures the in-band OSNR accurately by observing the output power of a fiber loop mirror filter, where the transmittance is adjusted by an embedded phase modulator driven by a low-frequency periodic signal. The measurement errors are less than 0.

Three-chip differential phase-shift keying maximum likelihood sequence estimation for chromatic-dispersion and polarization-mode-dispersion compensation.

We propose a novel three-chip differential phase-shift keying (DPSK) maximum likelihood sequence estimation (MLSE) for chromatic-dispersion (CD) and first-order polarization-mode-dispersion (PMD) compensation to extend the transmission reach of the DPSK signal. Such a technique searches the most probable path through the trellis for DPSK data sequence estimation by exploiting the phase difference between not only the adjacent optical bits but also the bits that are one bit slot apart. The proposed scheme significantly outperforms conventional two-chip DPSK MLSE in CD and PMD compensation.

Enhancing the monitoring sensitivity of DOP-based OSNR monitors in high OSNR region using off-center narrow-band optical filtering.

Recently, OSNR monitoring based on the measurement of degree of polarization (DOP) has attracted much attention, thanks to its simplicity and high efficiency. However, the OSNR monitoring sensitivity is quite poor in the high OSNR region, resulting in high estimation error and narrow dynamic range. In this paper, we propose and experimentally demonstrate a narrow-band off-center optical filtering technique for DOP-based OSNR monitors.

Polarization-interleaved WDM signals in a fiber optical parametric amplifier with orthogonal pumps.

We have demonstrated a polarization-interleaved wavelength-division multiplexed system with a two-orthogonal-pump optical parametric amplifier. The sensitivity has been improved by about 2dB compared to its counterpart with all channels co-polarized, with the same signal gain, which itself dramatically improved over the conventional two-parallel-pump OPA.

Simple PMD-insensitive OSNR monitoring scheme assisted by transmitter-side polarization scrambling.

Recently, degree of polarization (DOP) of light has been utilized as a parameter to monitor optical signal-to-noise-ratio (OSNR). However, in the presence of polarization-mode dispersion (PMD), when using DOP to determine OSNR, OSNR is always underestimated due to the depolarization effect induced by PMD. In this paper, we propose and experimentally demonstrate a simple PMD-insensitive OSNR monitoring technique based on DOP measurement.

Characterization of the performance of optical amplitude-shift keying-differential phase-shift keying orthogonally modulated signals.

We analytically investigate the signal performance of orthogonal modulation with slow amplitude-shift keying (ASK) data superimposed upon a high-speed differential phase-shift keying (DPSK) signal. The receiver sensitivities for both the ASK and the demodulated DPSK signals are formulated based on a probability model, and their dependence on the bit rate and the extinction ratio of the ASK data is theoretically investigated. A simple result of analysis of DPSK data performance has been derived.

Analysis of performance optimization in supercontinuum sources.

We have determined the cause of performance variation as a function of system parameters in supercontinuum (SC) sources in a normal-dispersion fiber by numerically analyzing the noise evolution of the SC. High-order nonlinearity was considered in such an analysis for the first time to our knowledge. We found that the evolution of noise along the SC fiber has two stages: First, the noise is governed by dispersion, which is different from that of the signal, whereas self-steeping leads to an asymmetric noise distribution across the spectrum.